The present invention relates to a rotor blade, and more particularly to an isolated trim tab for a rotor blade.
A rotary wing aircraft typically utilizes multiple rotor blades mounted to a rotor hub. A trim tab is a long, thin tab extending off the trailing edge of the rotor blade that can be bent along its length about a spanwise axis. Trim tabs change the effective airfoil shape and thus change the lift, drag, and bending-moment coefficients of the rotor blade airfoil at the local spanwise position of the tab. The ability to adjust these local airfoil parameters increases the amount of adjustment available to control global blade characteristics such as pitching moment slope, track, flutter stability, vibrations, and bending mode shapes.
Conventional trim tabs are typically either of an aluminum or composite structure. Aluminum trim tabs are often of a three-piece configuration in which a thin aluminum tab is sandwiched between two aluminum doublers mounted to a trailing edge of a rotor blade. The tab to doubler and doubler to blade bond lines are thin and consist of a cured film adhesive. Conventional aluminum trim tabs are readily adjustable in a field environment through a hand-held tool. The tool contains three rollers that clamp down on the tab and apply a pitching couple. The tool is rolled spanwise along the tab to bend it along its entire length.
Composite trim tabs are also of a three-piece configuration in which a thin thermoplastic-matrix trim tab is mounted between thermoset-matrix composite doublers. Adjusting the thermoplastic-matrix tab is relatively more difficult than an aluminum tab as heating is required to bend the tab. Composite trim tabs are therefore more difficult to adjust in a field environment.
Conventional trim tabs are located in low-strain regions of the blade as cracking of the tabs may otherwise occur if positioned at highly strained regions of the blade. Conventional aluminum trim tabs typically have a lower strain allowable than the trailing edge of the fiberglass/graphite laminate rotor blade. Thermoplastic-matrix composite trim tabs have an allowable strain similar to the trailing edge of the rotor blade, but may be relatively difficult to adjust.
The highest blade normal strains due to edgewise bending occur spanwise at the center of the blade and chordwise at the aft edge. Experience has shown that it would be desirable to position a trim tab at this location because some 3P blade vibrations may be reduced. Conventional trims tabs, however, rapidly fail at these central locations and may not provide a service life which make such positions feasible.
Accordingly, it is desirable to provide a rotor blade trim tab that is readily bendable in the field while achieving an acceptable service life when located at highly strained regions of the blade.